1. Field of the Invention
The present invention relates to an electron gun for cathode ray tubes and, more particularly, to a dynamic 4 polar electrode system in a pre-focusing electrode in an electron gun for cathode ray tubes, which can correct a horizontal focus deterioration and vertical moire of electron beams in a periphery of the screen.
2. Discussion of the Related Arts
In the prior art, respective electrodes of a in-line electron gun of a color cathode ray tube are sequentially spaced certain distances apart from a cathode in a direction of the screen vertical to a path of the electron beam. This permits the intensity of the electron beam emitted from each of the cathodes to be controlled by a bias voltage applied to each of the electrodes before arrival to the screen.
FIG. 1 illustrates a section of a general color cathode ray tube that includes a panel 1 forming a front part of the cathode ray tube, a funnel 2 having a front side fusion welded to the back of the panel 1 and backwardly converged, a neck part 3 formed at an end of the backwardly converge of the funnel. There are three cathodes 4 in an electron gun sealed in the neck part 3 arranged in-line horizontally for emitting thermal electron beams. The electron gun includes, sequentially starting from the cathodes towards the screen., a first electrode for controlling the electron beams, i.e., a controlling electrode 5, a second electrode for accelerating the electron beams, i.e., an accelerating electrode 6, third and fourth electrodes for pre-focusing the electron beams, i.e., pre-focusing electrodes 7 and 8, a fifth electrode having a dynamic four polar electrode part for focusing and correcting the electron beams, i.e., a focusing electrode 9, and a sixth electrodes interacting with the focusing electrode for forming a main lens and finally accelerating the electron beams, i.e., an anode 10. The electrodes are fixed in place by bead glass(not shown). There is a shield cup 11 disposed at one end of the anode 10, which faces the screen for prevention of electronic interference to the electron beams 12. Shield springs 13 fixed to the shield cup 11 are in contact with the graphite coated on inside surface of the funnel 2, thereby electrically connected to a cavity cap(not shown) on outer surface of the funnel 2. Each of the cathodes 4 is applied a voltage through a stem pin 14 one end of which is connected to respective cathode 4 and the other end is projected out of the neck part 3.
In order to compensate for the difference of thermal electron beam amounts emitted from each of the cathodes 4 caused by minute assembly errors between the controlling electrode 5 and the accelerating electrode 6 in assembly cathodes 4, each of the cathode is applied a voltage slightly different from the other. The controlling electrode 5 is grounded, a low voltage of 300.about.1000 V is applied to the accelerating electrode 6 and the fourth electrode 8 and a high voltage Eb of 27,000 V is applied to anode 10. The third electrode 7 and a first focusing electrode 91 adjacent to the fourth electrode 8 of the focusing electrode 9 which is divided into two are applied a static voltage Vsf from an intermediate voltage of 7000 V. The second focusing electrode 92 adjacent to the anode 10 is applied a dynamic voltage Vdf synchronous to a deflecting current and about 1000 V higher than the voltage to the first focusing electrode 91.
Accordingly upon application of currents to each of the cathodes 4 through the stem pins 14 on the electron gun for the conventional color cathode ray tube, a heater 15 in each of the cathodes 4 is heated to emit a electron beam from a surface of the cathode 4. The voltage on the accelerating electrode 6 accelerates the electron beams towards the panel, the pre-focusing electrodes 7 and 8 pre-focus the electron beams, and the focusing electrode 9 and the anode 10 finally focus and accelerate the electron beams. Thereafter, deflecting yokes 16 on outer circumference of the neck 5 at a transition of the panel 2 and the neck 5 deflects the electron beams to respective regions of the panel 1, to collide on a fluorescent surface 18 coated inside of the panel 1 through a color selective electron beam passing hole in a shadow mask 17 disposed on the inner side of the panel, thereby forming a pixel.
The electron beams 12 travelling along the path mentioned above are set so that they can make an exact convergence on the central portion of the panel 1 in case the electron beams are not deflected. However, in case the electron beams are deflected, the convergence of the electron beams can be mismatched because, in general, of a difference of the curvature between central and peripheral portions of the panel and the in-line configuration of the electron gun, which causes the electron beams 12 emitted from each of the cathodes to travel a distance farther than a distance to the central portion of the screen when the electron beams 12 are deflected to a periphery of the screen. In general, this mismatch of the convergence can be corrected by devising the deflecting yokes 16 which deflect the electron beams to form a nonuniform magnetic field.
The nonuniform magnetic field is a magnetic field consisting of a pin cushion type magnetic field formed by a saddle type horizontal winding of coil, of the coil wound on the deflecting yoke, and a barrel type magnetic field formed by a troidal vertical winding of coil. The pin cushion type magnetic field deflects and slightly focuses the electron beams in the horizontal direction and the barrel type magnetic field deflects and focuses the electron beams in the vertical direction. However, the horizontal slight focusing capability of the pin cushion type magnetic field and the vertical focusing capability of the barrel type magnetic field combine in expanding the electron beams excessively in the horizontal direction and focusing the electron beams excessively in the vertical direction in a periphery of the screen. This results in formation of a high density, transversely elongated core and a vertical haze which is a low density dispersion of an image. Such excessive horizontal expansion and vertical haze are corrected by a first dynamic four polar electrode part A provided in the first and second focusing electrode 91 and 92.
This will be explained in more detail with reference to FIGS. 2A and 2B. FIG. 2A illustrates a cross section of an in-line type dynamic electron gun for a color cathode ray tube having the first dynamic four polar electrode part in the focusing electrodes, and FIG. 2A illustrates a section across I--I line in FIG. 2B.
In FIGS. 2A and 2B, the first dynamic four polar electrode part includes three electron beam pass-through holes 921 formed in the second focusing electrode 92 at cathode side, horizontal partition walls 922 on upper and lower sides of the three electron beam pass-through holes 921, a rim 912 having an electron beam pass-through hole 911 for passing the three electron beams in common formed on the first focusing electrode 91 at the screen side, and an inner electrode 93 having three electron beam pass-through holes 931 for passing the three electron beams inside of the first focusing electrode 91. Burring parts 923 and 933 are provided around the electron beam pass-through hole 931 in the inner electrode 93 and the electron beam pass-through hole 921 at the cathode side of the second focusing electrode 92. The burring parts 923 and 933 are projected to the cathodes 4 and the screen in directions opposite to each other. As shown in FIG. 2B, the horizontal partition walls 922 has curved parts 922A at upper and lower sides of the electron beam pass-through holes 921 of the second focusing electrode 92, and straightened parts 922B are at the parts of connecting the electron beam pass-through holes 921 and outer sides of the electron beam pass-through holes 921.
The first focusing electrode 91 is applied a static voltage Vsf of 7000 V and the second focusing lens 92 is applied a dynamic voltage Vdf about 1000 V higher than the static voltage to the first focusing electrode 91 and synchronous to a deflection signal depending on an extent of deflection of the electron beams. The four polar dynamic lens is formed between the first and second focusing electrodes 91 and 92 by a voltage difference of the static voltage Vsf to the first 10 focusing electrode 91 and the dynamic voltage Vdf to the second focusing electrode 92. Particularly, since the horizontal partition walls 922 are provided at upper and lower sides of the second focusing electrode 92 to which a higher voltage is applied to focus the electron beams slightly, a vertical focusing force of the electron beams in the periphery of the screen is weakened because a vertical slight focusing force for the electron beams acts intensely. This eliminates haze and improves resolution in the periphery of the screen as shown in FIG. 3B because the intense vertical slight focusing force compensate for the excessive focusing caused by the nonuniform magnetic field of the deflection yokes.
However, since the first dynamic four polar electrode part synchronous to the dynamic voltage of the deflection yokes has been designed taking only the deterioration of the electron beams caused by the nonuniform magnetic field of the deflection yokes into consideration, but not the relationship of the pre-focus lens, the pre-focus electrode can not provide an optimal cross-over point diameter and pre-focusing angle to the main lens. Accordingly, the horizontal focusing deterioration caused by horizontally enlarged and vertically reduced spot in a periphery of the screen can not be eliminated completely and a limitation in correction of the deterioration occurs. In addition because of the particular reduction of the vertical spot size in a low current range in deflection of the beams, moire is caused in a vertical direction by a deflection current that further deteriorates the resolution.